(1) Field of the Invention
The invention relates to the field of the production of flat glass and relates more specifically to a method for cutting a continuous glass ribbon.
A continuous glass ribbon is to be understood as meaning a glass ribbon coming from a continuous forming process, particular a float process or a glass rolling process.
(2) Description of Related Art
In general, a continuous glass ribbon is produced from a glassmaking furnace into which vitrifiable raw materials are introduced and melted to form a viscous glass. This viscous glass is fed to a forming tool, such as a float for example, where the viscous glass is poured out onto a bath of molten tin held in a reducing atmosphere, or a rolling mill. This then yields, at the output from the forming tool, a continuous glass ribbon that has to be cut for later applications, particularly for applications in the field of building and/or the motor vehicle.
The traditional method used for cutting glass consists first of all in annealing the glass ribbon then in cutting it in the transverse direction into plates and then cutting the edges off these plates in the longitudinal direction.
The term “annealing” is given, in the usual way, to the operation that consists in causing the glass ribbon to undergo a controlled cooling cycle so as to progressively relieve the stresses induced by the forming step.
The “transverse direction” is the term given to an axis roughly perpendicular to the axis of travel of the glass ribbon leaving the forming tool.
The term “longitudinal direction” is given to an axis roughly parallel to the axis of travel of the glass ribbon leaving the forming tool.
The term “plates” is given to elements of the glass ribbon once it has been cut in the transverse direction.
The traditional method of cutting a continuous glass ribbon involves four main steps after the glass ribbon has been annealed:
first step: longitudinal marking, particularly using a cutting wheel, of a roughly continuous notch along each of the edges of one side of the ribbon in the longitudinal direction. This marking is performed a few centimeters to a few tens of centimeters away from the outside edge of the ribbon. Its purpose is to create surface defects on the glass to prepare for the subsequent cutting-off of the edges of the ribbon, an operation known as “edge trimming”.
second step: marking a continuous notch along one side of the ribbon in the transverse direction. This marking is performed in particular with a cutting wheel which moves at an angle to the axis of travel of the glass ribbon, so as to obtain on the ribbon a marking perpendicular to the axis of travel.
third step: division of the ribbon at the transverse marking to form a plate from the continuous ribbon. This step is known as “transverse breakage”. The plate thus formed is then generally transported by rolls at a rate very much higher than the velocity at which the continuous ribbon travels.
fourth step “edge trimming”: separation of the region between the outside edge of the plate and the marked line, the region termed the “edge”, from the remainder of the plate to obtain a plate which constitutes a finished or part-finished product which is then stored, generally roughly vertically, then dispatched to the customer on easels.
This fourth step, the edge trimming, first of all comprises a phase in which a shock is produced, for example using a metal wheel, at the top of the plate, at the location where the longitudinal marking was made. The shock creates an addition of energy such that a crack develops and follows the fault line of the marking. A crack generated under these conditions propagates very quickly, particularly at the speed of sound, namely about 1 000 m per second. The edge strip becomes detached therefore from the plate and is removed. In the case of thick glass, the propagation of the crack may be assisted with the use of hammers, particularly pneumatic ones, that generate regular impacts on the underside of the plate.
The transverse-breakage technique is described in particular in patent U.S. Pat. No. 4,072,259.
The edge trimming technique has been improved many times, these being set out, for example, in patents U.S. Pat. No. 4,196,830, U.S. Pat. No. 4,285,451, U.S. Pat. No. 4,466,562 and U.S. Pat. No. 4,489,870.
Those patents describe improvements to the conventional technique mentioned hereinabove, particularly and respectively the possibility of easing the edge trimming by local heating/cooling of the glass, of cutting off the edges from two parallel longitudinal markings on the same side, or from two longitudinal markings facing each other, on each side of the glass, of developing means to apply a moment to the edge strip by applying a load close to the outside edge of the edge strip so as to open up the crack starting from the marking.
It will be noted that all these improvements relate to means that come into effect after the continuous ribbon has been cut to form plates, that is to say after the transverse breakage.
These traditional edge-trimming techniques are relatively well suited to the industrial processes used to produce glass of standard thickness, particularly between 3 and 8 mm thick.
However, numerous problems arise in cutting the edges off when the thickness of the ribbon is either smaller than or greater than the thickness used in standard glass.
In the case of thin glass, particularly where the thickness is less than or equal to 1 mm, splintering is very often observed (see, for example: “lateral cracks”, Fractography of Glass—Bradt, R. C.; Tressler, R. E., Plenum Press, New York, 1994) on the trimmed edge face using traditional techniques. This type of defect detracts considerably from the mechanical properties of the glass. Such defects are unacceptable to the customer and lead to very high reject rates.
In the case of thick glass, particularly where the thickness is greater than or equal to 10 mm, particularly greater than or equal to 15 mm, edge faces trimmed using the traditional techniques are found that have numerous defects. On the one hand, the edge faces are not generally perpendicular to the largest sides (the sides corresponding to the horizontal faces of the continuous glass ribbon) but cut at a bevel.
Numerous defects that may cause mechanical weakness are also found, particularly those known by those skilled in the art as flakes, burrs, broken corners, and branched cracks.
This type of defect needs to be eliminated so that the plates can be used as finished products, particularly in applications where it is desirable to toughen the glass because the toughening operation very highly stresses defects in the glass and often leads to breakage if defects are present.
It should be noted that most production of thick glass is intended for applications where the glass needs to be toughened.
To eliminate such defects introduced by the edge trimming using traditional techniques, a shaping step is performed, this consisting in removing material, particularly by abrasion, so as to straighten up the edges and obtain a face free of defects likely to lead to breakage during toughening. This operation is lengthy and expensive.